Integrated circuits (ICs) comprising many tens of thousands of devices including field effect transistors (FETs) and other devices are a cornerstone of modern microelectronic systems. With the scaling of semiconductor technology, there is a demand for high voltage (e.g. 6-40V) range devices such as high voltage transistors with minimum pitch for example in applications such as Liquid Crystal Display (LCD) driver market.
FIG. 1 is a cross-sectional view of a known high voltage NMOS transistor 100. The NMOS transistor 100 comprises a substrate 102, such as a P-type body, with shallow trench isolation (STI) structures 104 formed therein. A gate insulator 106 is provided over the substrate 102 and a gate 108 formed above a portion of the gate insulator 106.
Lightly doped N-type drift regions (110a, 110b) are formed in the substrate 102 on opposing sides of the gate 108 and in partial overlap with the gate 108. A heavily doped N-type source region 112a is formed within one of the lightly doped drift regions 110a and a heavily doped N-type drain region 112b is formed within the other lightly doped drift region 110b. The heavily doped source and drain regions (112a, 112b) are formed to a shallower depth than the lightly doped drift regions (110a, 110b). The heavily doped source and drain regions (112a, 112b) are separated from the lower edge of the gate 108 by an STI structure 104. The NMOS transistor 100 channel region 105 is located along the surface of the substrate 102 between lightly doped N-type drift regions (110a, 110b).
During the operation of high voltage transistors, a high voltage is applied to the source or drain regions (112a, 112b). The transistor's ability to withstand such high voltage application is largely dependent on the distance between the heavily doped source and drain regions (112a, 112b) and the channel region 105, also referred to as the length of the drift region. A longer drift region translates into a higher breakdown voltage. Unfortunately, it is difficult to maintain a long drift region and hence high breakdown voltage while reducing the size of the high voltage transistor.
As a result, semiconductor structures that provide high voltage transistors at decreased transistor pitch and methods for fabrication thereof are desirable.